1. Field of the Invention
The present invention relates to a method for manufacturing a bottle-shaped can, of which a can trunk, a shoulder portion and a neck portion having a threaded portion are integrally formed, of a metallic sheet having a thickness of 0.1 to 0.4 mm.
More particularly, the invention relates to a forming method for forming especially the shoulder portion into a smooth and beautiful slope not in a step shape or a shape having a step mark left, when the bottle-shaped can is to be manufactured by working the bottom side of the can formed into a bottomed cylindrical shape, to form the shoulder portion having an inclined annular face and a diametrically small cylindrical neck portion integrally.
2. Related Art
As beverage cans for various soft drinks or beer, there are generally employed the DI cans (Drawn 5 and Ironed cans), of which the can trunk (or side wall portion) and the can bottom are integrally formed by drawing and ironing a metallic sheet such as an aluminum alloy sheet or a surface-treated steel sheet.
Specifically, this DI can has its body formed by integrally forming the bottom portion having a shape of a high pressure resistance and a trunk portion thinned by the drawing and ironing workings and by necking in the open upper end of the trunk portion to reduce a diameter of the open upper end. The can body is filled with a drink such as a soft drink or beer, and the diametrically reduced open upper end is seamed with an easy open end (i.e., an end sheet having an easy opening) having a smaller diameter than that of the trunk. These cans are shipped as canned drinks.
As disclosed in WO 81/01259, on the other hand, there is also practiced the bottomed cylindrical can which is formed to have a thinner trunk wall than a bottom wall by drawing and re-drawing (or bending and extending at the re-drawing time) the surface-treated steel sheet laminated on its two sides with a thermoplastic resin film. The can thus manufactured is necked in like the DI can so that it may be used as the beverage can.
As the containers for various soft drinks, on the other hand, there have been employed in recent years the bi-oriented molded container made of a polyethylene terephthalate resin (i.e., the PET bottle). Accordingly, there have been manufactured various beverages contained in the PET bottles which can be repeatedly sealed with threaded caps.
These beverage PET bottles have an advantage over the above-described can containers for beverages in that the PET bottles can be repeatedly sealed with the caps. However, the PET bottles are in considerably lower states than those of the can containers in the recycling ratio for collecting and recycling the resources. Therefore, it has been investigated to enhance the conveniences of the can containers by adding re-sealing function to the can containers having the high resource recycling ratio.
In the prior art, there are disclosed in Japanese Patent Laid-Open No. 10-509095 (WO96/15865) several types of bottle-type DI cans having shapes similar to those of the PET bottles, i.e., the DI cans which have threaded neck portions to be screwed and closed with the threaded caps.
These DI cans are classified into: the type in which an end sheet to be seamed on the open upper end of a can trunk is formed integrally with a threaded neck portion; the type in which the threaded neck portion is integrally formed by reducing the diameter of the open upper end side of the can trunk stepwise by the neck-in working; and the type in which the diametrically small neck portion and the shoulder portion having a slope are formed by drawing the bottom portion side (or the end wall portion) of a cup at multiple steps, in which the trunk portion of the cup is then ironed into a thin trunk portion and in which a threaded portion is formed in the neck portion whereas the can end is seamed on the open end of the trunk portion. In the above-specified Laid-Open, there are disclosed not only the structures of the bottle-shaped cans of the individual types but also the forming methods.
According to the disclosure of Japanese Patent Laid-Open No. 58-47520, on the other hand, at the time of drawing the can trunk, the bottom portion is drawn into a convex stepped shape, and this convex stepped shape is re-drawn at a subsequent ironing time, to form a convex stepped portion having a diametrically small cylindrical neck portion and a square shoulder portion in the bottom portion (or the end wall portion) of the DI can. This neck portion is threaded and sealed with the threaded cap. After this DI can was filled with beverage from the end opening of the trunk portion, this end opening is sealed by seaming the can end.
In Japanese Patent Laid-Open No. 64-62233, moreover, there is disclosed that the DI can drawn and ironed is pressed (or drawn) at its bottom to form a diametrically small cylindrical neck portion and a frusto-conical shoulder portion (having a shape of a frustum of a cone) and that a thread is then formed in or a threaded cylindrical portion is mounted on the neck portion.
Of the aforementioned bottle-shaped cans which can be sealed again with the threaded cap, the can of the type in which the threaded neck portion is formed integrally with the end sheet is formed at its body into the DI can or a bottomed can such as the DTRD can (Drawn, Thin and Re-Drawn) formed by being drawn and bent/extended (or stretched) or the can formed by being bent/extended (or stretched) and ironed. The can trunk is filled with a content such as beverage, and then the open upper end of the can trunk is seamed and sealed with the end sheet having the threaded neck portion formed integrally therewith. According to the bottle-shaped can of this type, therefore, the can body has a shape substantially identical to that of the existing general can, and enjoys an advantage that few changes are required in the filling facilities to suppress the cost for the facilities.
In the bottle-shaped can of this type, however, the end sheet seaming portion is located in the upper portion of the can to raise problems that dust is liable to accumulate in the concave portion inside of the seamed portion, and that the seamed portion itself protrudes to deteriorate the appearance.
In the bottle-shaped can of the type in which the neck portion is formed not at the end sheet but integrally at the upper end portion of the can body, on the other hand, the upper end portion is stretched thin by a similar working as the can body is drawn and ironed or bent and extended. Considering the later step of working the neck portion, therefore, the upper end portion of the can body may be worked so relatively thick as to make the extension of the material less than the lower portion. Since the neck portion has a considerably smaller diameter than that of the can body, however, the diametrical reduction ratio for forming the neck portion is so large that the neck portion is difficult to constrict all at once by enlarging one drawing rate. In addition, it is desirable to reduce the cap size so as to lower the cost for the material and accordingly to reduce the diameter of the neck portion more than the external diameter of the can body. In order to satisfy these desires, it is necessary to make the diametrical reduction ratio more for forming the neck portion by drawing the open upper end of the can body, and this necessity requires multiple steps of neck-in workings.
For example, the can to be relatively frequently used as the drink can for beer has a trunk diameter of 66 mm (of 211 diameter), and twenty to thirty necking steps are required if the neck portion of such can is to be necked in to a diameter of 25.4 mm. Thus, in the bottle-shaped can having the neck portion formed by constricting the open upper end of the can body, a number of necking machines are required to raise the cost for the facilities, and the increase in the number of working steps makes it frequent to damage or deform the can thereby to lower the quality of the can.
In the bottle-shaped can of the type in which the shoulder portion and the neck portion are formed by working the can bottom, on the contrary, the can bottom portion or the portion to be formed into a portion of the shoulder portion and the neck portion is hardly affected by the working to form the can so that the working is applied to the portion having no work hardening and having a thickness substantially equal to that of the original metallic sheet. When the can bottom is to be drawn, moreover, its diameter can be reduced while being unwrinkled. As compared with the case in which the neck portion is formed by necking in the upper portion of the aforementioned can trunk, therefore, one drawing extent can be increased to reduce the diameter more by one step thereby to reduce the number of steps drastically for forming the neck portion.
Although one drawing extent can be made larger than that of the neck-in working, however, there is a limit to the drawing ratio (i.e., the reduction ratio of one drawing extent). The limit of the drawing ratio in the drawing case with the unwrinkling function is more or less different for the materials and is about 1.5 for a beverage can of a metallic sheet such as a beer can. When a neck portion (having diameter of 25.4 mm) of a diametrically small cylindrical shape is formed by drawing the flat bottom of the bottomed cylindrical DI can (having a diameter of 66 mm), it is necessary to repeat the drawing step three or four times.
As a method for forming the diametrically small neck portion integrally with the bottom side of the DI can, there has been disclosed in Japanese Patent Laid-Open No. 58-47520 a method for manufacturing the DI can having a diameter of 67.83 mm and made of a tinned steel sheet. At a re-drawing step, the bottom portion of the can is re-drawn to form a convex stepped portion having a diameter of 26 mm. This convex stepped portion of the can bottom is re-drawn at the final stroke of the subsequent ironing step, to form the shoulder portion having the so-called “square shouldered portions” and the cylindrical neck portion having a height of 6 mm and a diameter of 16 mm.
In Japanese Patent Laid-Open No. 64-62233, there is disclosed a method for forming a frusto-conical shoulder portion and a cylindrical neck portion by pressing (or drawing) the bottom of the DI can formed into the bottomed cylindrical shape.
According to the former method, however, the height of the neck portion is too short at 6 mm to form a threaded portion capable of retaining sufficient sealing properties. According to the latter method, although the drawing step of multiple stages is not shown in the Drawings of the Specification, it is apparent from FIGS. 2 and 3 that the can trunk has a thickness of two to three times of that of the bottom end, and it is understood from the description of the Specification anticipating the threading of the neck portion that a relatively thick (e.g., about 0.6 to 1.5 mm) aluminum alloy sheet or stainless steel sheet is employed as the trunk material. When this thick material is employed, less wrinkles are formed by the drawing even at a high drawing ratio. Even if so, many drawing steps have to be repeated for forming the diametrically small cylindrical neck portion and the frusto-conical shoulder portion (or a truncated conical shoulder portion). Although the wrinkling can be suppressed, therefore, step shapes or many-circumferential marks according to the number of drawing steps are left on the shoulder portion. These step shapes are identical to a plurality of convex and concave steps or ribs, as shown in FIG. 28 of Japanese Patent Laid-Open No. 10-509095, or a plurality of convex and concave steps or circumferential beads 108 shown in FIG. 32.
Here, the problems of the step shapes at the multiple drawing steps by the pressing or the circular shock marks formed when those step shapes are smoothed are described, as follows, in Japanese Patent Laid-Open No. 55-107638 disclosing a method for manufacturing a cup-shaped end for small-sized beer bottles.
In the prior art, after the multiple drawing steps for drawing the can end into a cup shape, a stretching (or a final forming) is performed to finish the end into a predetermined smooth cup shape and to size the cover. By the multiple pre-drawing steps in these working steps, a plurality of concentric and annular convex portions are formed on the outer face of the cup-shaped end body. These are the portions which are formed by the outer circumference edges of the leading end of a punch having different diameters. These annular convex portions have to be turned backward of their warped direction so that they may be eliminated. In this case, the surface structure of the material, as once extended, receives an inverse compression stress so that the aforementioned shock marks are formed to lower the commercial value seriously.
These shock marks look like streaks not only to degrade the appearance but also to lower the corrosion resistance. On the other hand, the shock marks also occur on the inner face of the end body to separate the coating and cause the corrosion at the shock marks when the material is exemplified by the aluminum alloy sheet having the coating of an epoxy resin on the inner and outer faces, so that the can is unsuited as the can for confining food. When the aluminum alloy sheet is employed for the food can, therefore, the can has to be surface-treated, after pressed, by oxidizing or coating it so that an increase in cost cannot be avoided.
In the invention disclosed in Japanese Patent Laid-Open No. 55-107638, therefore, it is intended to prevent the shock marks from appearing, by leaving the annular convex and concave portions formed by the multiple drawing steps on the cup-shaped end body.
If these annular convex and concave portions are on the upper face of the container, however, it cannot be said that the appearance is excellent. In the container of this shape, on the other hand, dust is liable to accumulate in the concave portions while the container is displayed in the shop and cannot be easily wiped off, to raise a problem that the appearance is degraded.
By repeating the drawing of the flat bottom of the DI can three or four times, as described before, the diametrically small cylindrical neck portion and the shoulder portion having the slope can be formed integrally with the can body, to provide a bottle-shaped can having a shape resembling the PET bottle having a round transverse section, as employed as the ordinary beverage container. At the individual drawing steps of the forming steps, the unwrinkling has to be performed with individual tools. Therefore, the ring-shaped and the step-shaped portions corresponding to the shape of the inner circumference end edges of the drawing die are so formed at the portion or the shoulder portion of the bottle-shaped can according to the number of drawing (or re-drawing) steps, and a clear boundary line is formed between the cylindrical portion and the slope portion. In order to eliminate those step-shaped portions and the boundary line thereby to provide a smooth slope, it is conceivable to perform the pressing working by using a pair of reforming tools having a curved slope of a domed longitudinal section or a slope of a straight longitudinal section, thereby to reform the shoulder portion which is formed profiling the surface shapes of those forming tools. Even with this pressing working, however, the step-shaped portions and the clear boundary line between the cylindrical portion and the slope portion are left as the forming marks of ring shape to degrade the appearance.
This will be described in more detail. According to the method for manufacturing the diametrically small cylindrical portion and the sloped shoulder portion gradually by repeating a plurality of drawing steps, as described above, the portion, as formed before re-drawn as the circumferential boundary line between the diametrically small cylindrical portion and the slope, is left in an apparent state as the circular mark in a portion of the sloped shoulder portion leading downward to the cylindrical portion which has been re-drawn into a smaller diameter.
If the drawing working is performed four times, three circular step portions (or three step-shaped portions) or boundary line marks are clearly left on the shoulder portion. These circular step portions or marks cannot be eliminated even after the shoulder portion was reformed.
Specifically, the portion, as has been the boundary line between the diametrically small cylindrical portion and the slope, is clearly left as the mark of the circular step portion or the boundary line on the shoulder portion which is newly formed at the subsequent drawing step, and this mark cannot be eliminated in the prior art even by reforming the shoulder portion.
The circular mark thus far described will not seriously affect the function of the container but will make an important point as the commercial goods. Specifically, the image of a commodity is expressed by the appearance of the container so that the beverage maker always demands for a design to stimulate the purchasing wills of the consumers. When the can is to be manufactured, therefore, it is an important point of design to form the shoulder portion from the neck portion to the can trunk into a smooth and beautiful curved slope of a domed longitudinal section or a smooth and beautiful slope of a straight longitudinal section. Hence, the aforementioned forming mark is earnestly desired to disappear because it is a fatal defect in the design.
Here in Japanese Patent Laid-Open No. 10-509095, especially in its FIGS. 18 to 27, there is disclosed a method for forming a cup trunk portion (or a side wall portion) into a diametrically small and thin cylindrical trunk portion. According to this method, the cup, as drawn from a blank punched out from a metallic sheet, is first drawn at its bottom portion repeatedly by several times (preferably, three or more) to form a diametrically small cylindrical neck portion. Next, the neck portion of the bottom portion is bulged at the domed shoulder portion. After this, the cup trunk portion (side wall portion) is re-drawn and ironed to form the diametrically small and thin cylindrical trunk portion.
According to the disclosed method, however, when the can trunk is formed, the metallic sheet material of the neck portion, as formed at the bottom of the cup, is pulled through the shoulder portion into the trunk portion of the can trunk as the metallic sheet material moves from the side wall portion of the cup to the thin trunk portion of the can trunk. As a result, the cylindrical shape of the neck portion, as formed at the bottom portion of the cup, cannot be kept in the initial shape so that the cylindrical vertical wall of the neck portion turns into a frusto-conical tapered wall. In this state, there arises a disadvantage that a predetermined thread cannot be formed at the step of threading the neck portion. In order to keep the sealing performance between the neck portion and the threaded cap, therefore, there arises a problem that the neck portion and the shoulder portion have to be reformed so as to raise the cylindrical neck portion as the vertical wall from the shoulder portion bulged in the domed shape.